Technical Field
The present invention relates to an electric motor which can be used as a main machine for electric vehicles and also can be used in various applications including industrial motors and generators,
Related Art
In electric vehicles, there is mounted an electric motor which generates power as a man-machine motor, For the main-machine motor, the motor is frequently confronted with a severe running condition when the vehicle climbs a hill. Even when a motor whose high-efficiency and high power factor are given in its operating range for high frequency of usage is used as the main-machine motor, the motor still has a problem. The problem is that the power factor drops down to about 0.6 and a copper loss becomes larger in a driving mode which needs lower rotation speeds and larger amounts of torque in a hill-climbing run of the vehicle. For example, compared with current phase gained at a power factor of 1.0, the current phase at a power factor of 0.6 is −53.13 degrees, and the copper loss increases up to an amount which is 2.777 times (=1/(0.6×0.6)). The current capacity of an inverter increases up to an amount which is 1.666 times (=1/0.6), When the motor loss is larger in an operating mode requiring larger amounts of torque, the motor becomes larger in its size, thereby increasing production cost thereof. Meanwhile, the main-machine motor mounted in, for example, electric vehicles need a wider torque characteristic ranging from a basic rotation speed to a high rotation speed.
FIG. 13 is a lateral sectional view illustrating a rotor 279 equipped with a diode 27A and field windings 271-278 electrically and serially connected to the diode. The field windings 271 and 272, which are wound as illustrated in the drawing, magnetically excite an N-magnetic pole 27F. A reference 27G shows a coil end portion of the field windings 271 and 272. Similarly, the field windings 273, 274; 275, 276; and 277, 278 are wound in series to each other so as to provide predetermined directional currents in the rotor 279. FIG. 13 also shows an equivalent circuit of electrical connections among the field windings and the diode 27A.
In this rotor 279, a stator-side field current Isf (not shown) increases from zero, each of the rotor magnetic poles is magnetically excited to produce a field magnetic flux φf (not shown) which increases proportionally to an increase in the field current Isf. During the increase in the field magnetic flux φf, a rotor field current If will not follow due to a negative voltage Vf (not shown) generated through the rotor field windings in FIG. 13, (a). The field current Isf on the stator side starts to reduce, a positive voltage Vf (which is expressed by “−dφf/dt”) is generated proportionally to the number of windings, whereby the field current If starts to flow in the forward direction of the diode 27A. Further, at the moment when the stator-side field current Isf becomes zero, the rotor field current If still maintains generation of the field magnetic flux φf. However, the field magnetic flux φf reduces with time. Hence, supply of the stator-side field current Isf in a pulsed manner makes it possible to maintain the field magnetic flux φf at an approximately constant amount. Incidentally the field current Isf can be refereed as a d-axis current component.
However the motor structure shown in FIG. 13 has various drawbacks. One drawback is that the field current Isf becomes larger when the motor is driven at lower speeds and larger amounts of torque. As a result, it is difficult to largely reduce the copper loss of the stator. Another drawback is about the size of an inverter to drive the motor. That is, since the inverter has to perform switchovers among larger amounts of currents, this becomes an obstacle in reducing the size of the inverter. Another drawback comes from the intermittent control of the field current Isf. This control results in generation of ripples of torque, vibration and noise. These drawbacks are due to the fact magnetic energy of the field can be supplied from the stator to the rotor 279 but power cannot be supplied to the rotor like electric transformers. In addition, in the structure shown in FIG. 13, spatial harmonic components of the motor generate voltages across the field windings, but advantages thanks to these voltages are limited in a desired range where rotation speeds are lower which need larger amounts of torque.
Another main-machine motor is exemplified in JP-B-5363913, in which a three-phase full-pitch winding, and distributed-winding motor is provided. However, this motor has drawbacks which include a longer length of the coil ends which are due to its complex structure, a lower space factor of the windings in slots, and higher production costs.
[PTL 1] JP-B-5363913
[PTL 2] JP-A-2004-166476